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<meta name="description" content="是一个长度为 0 且占内存的空字符串，在内存中分配一个空间，可以使用 Object 对象中的方法。 例如： .toString() 等。 null 是空引用，表示一个对象的值，没有分配内存 ，调用 null 的字">
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			<h1>Java中容易混淆的空字符串和null</h1>
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                    <span class="pre left"><span class="icon">&lt;</span> <span class="text-brief text-brief-pre">上一页</span><a href="http://c.biancheng.net/view/5808.html">Java字符串比较</a></span>
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			<div id="arc-body"><span style="color:#b22222;">“”是一个长度为 0 且占内存的空字符串，在内存中分配一个空间，可以使用 Object 对象中的方法。</span>例如：<code>“”.toString()</code> 等。<br>
<br>
<span style="color:#b22222;">null 是空引用，表示一个对象的值，没有分配内存</span>，调用 null 的字符串的方法会抛出空指针异常。例如如下代码：
<p class="info-box">
	String str = null;<br>
	System.out.println(str.length());</p>
new String() 创建一个字符串对象的默认值为 ""，String 类型成员变量的初始值为 null。<br>
<br>
空字符串 "" 是长度为 0 的字符串。可以调用以下代码检查一个字符串是否为空：
<p class="info-box">
	if (str.length() == 0)</p>
或
<p class="info-box">
	if (str.equals(""))</p>
空字符串是一个 Java 对象，有自己的串长度（0）和内容（空）。不过，String 变量还可以存放一个特殊的值，名为 null，这表示目前没有任何对象与该变量关联。要检查一个字符串是否为 null，要使用以下条件：
<p class="info-box">
	if (str == null)</p>
有时要检查一个字符串既不是 null 也不为空串，这种情况下就需要使用以下条件：
<p class="info-box">
	if (str != null &amp;&amp; str.length() != 0)</p>
注意：首先要检查 str 不为 null。如果在一个 null 值上调用方法，会出现错误。<br>
<br>
示例如下：
<div class="snippet-container" style="undefined;"><div class="sh_bright snippet-wrap"><div class="snippet-menu sh_sourceCode" style="display:none;"><pre><a class="snippet-copy sh_url" href="http://c.biancheng.net/view/vip_7052.html#" style="display: none;">复制</a><a class="snippet-text sh_url" href="http://c.biancheng.net/view/vip_7052.html#">纯文本</a><a class="snippet-window sh_url" href="http://c.biancheng.net/view/vip_7052.html#">复制</a></pre></div><pre class="java sh_java snippet-formatted sh_sourceCode"><ol class="snippet-num"><li><span class="sh_keyword">public</span> <span class="sh_keyword">static</span> <span class="sh_type">void</span> <span class="sh_function">main</span><span class="sh_symbol">(</span>String<span class="sh_symbol">[]</span> args<span class="sh_symbol">)</span> <span class="sh_cbracket">{</span></li><li>    <span class="sh_usertype">String</span><span class="sh_normal"> </span>str1 <span class="sh_symbol">=</span> <span class="sh_keyword">new</span> <span class="sh_function">String</span><span class="sh_symbol">();</span></li><li>    <span class="sh_usertype">String</span><span class="sh_normal"> </span>str2 <span class="sh_symbol">=</span> <span class="sh_keyword">null</span><span class="sh_symbol">;</span></li><li>    <span class="sh_usertype">String</span><span class="sh_normal"> </span>str3 <span class="sh_symbol">=</span> <span class="sh_string">""</span><span class="sh_symbol">;</span></li><li><span style="display:none;">&nbsp;</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str3<span class="sh_symbol">.</span><span class="sh_function">length</span><span class="sh_symbol">());</span> <span class="sh_comment">// 空字符串""的长度为0</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str2<span class="sh_symbol">.</span><span class="sh_function">length</span><span class="sh_symbol">());</span> <span class="sh_comment">// 抛出空指针异常</span></li><li><span style="display:none;">&nbsp;</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str1<span class="sh_symbol">);</span> <span class="sh_comment">// 输出""</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str1 <span class="sh_symbol">==</span> str2<span class="sh_symbol">);</span> <span class="sh_comment">// 内存地址的比较，返回false</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str1<span class="sh_symbol">.</span><span class="sh_function">equals</span><span class="sh_symbol">(</span>str2<span class="sh_symbol">));</span> <span class="sh_comment">// 值的比较，返回false</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str2 <span class="sh_symbol">==</span> str3<span class="sh_symbol">);</span> <span class="sh_comment">// 内存地址的比较，返回false</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str3<span class="sh_symbol">.</span><span class="sh_function">equals</span><span class="sh_symbol">(</span>str2<span class="sh_symbol">));</span> <span class="sh_comment">// 值的比较，返回false</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str1 <span class="sh_symbol">==</span> str3<span class="sh_symbol">);</span> <span class="sh_comment">// 内存地址的比较，返回false</span></li><li>    System<span class="sh_symbol">.</span>out<span class="sh_symbol">.</span><span class="sh_function">println</span><span class="sh_symbol">(</span>str1<span class="sh_symbol">.</span><span class="sh_function">equals</span><span class="sh_symbol">(</span>str3<span class="sh_symbol">));</span> <span class="sh_comment">// 值的比较，返回true</span></li><li><span class="sh_cbracket">}</span></li></ol></pre><pre class="snippet-textonly sh_sourceCode" style="display:none;">public static void main(String[] args) {
    String str1 = new String();
    String str2 = null;
    String str3 = "";

    System.out.println(str3.length()); // 空字符串""的长度为0
    System.out.println(str2.length()); // 抛出空指针异常

    System.out.println(str1); // 输出""
    System.out.println(str1 == str2); // 内存地址的比较，返回false
    System.out.println(str1.equals(str2)); // 值的比较，返回false
    System.out.println(str2 == str3); // 内存地址的比较，返回false
    System.out.println(str3.equals(str2)); // 值的比较，返回false
    System.out.println(str1 == str3); // 内存地址的比较，返回false
    System.out.println(str1.equals(str3)); // 值的比较，返回true
}</pre></div></div>
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